I. Field of the Disclosure
The technology of the disclosure relates generally to battery monitoring systems for battery power systems, such as uninterrupted power supplies (UPSs).
II. Background
An industrial system may rely on an uninterrupted power supply (UPS) to provide backup power in the event of a primary power system failure. The UPS may be provided in the form of a number of lead acid battery cells electrically connected in series. A battery charger is provided that keeps the battery cells charged in the event backup power is needed from the battery cells. However, each battery cell will eventually fail. For example, lead acid batteries may lose the ability to accept a charge when discharged over time due to sulfation. A battery containing one or more failed battery cells may be unable to power the industrial system at specified battery operating voltages, at specified battery operating currents, and/or for specified battery time durations.
Accordingly, an industrial system may employ a battery monitoring system to monitor the state-of-health (SOH) of battery cells in a backup power supply. The SOH of the battery cells can be used by service personnel to replace failing or underperforming battery cells to ensure the UPS function provided by the battery cells will deliver back-up power when needed. In this regard, the state-of-health (SOH) of a battery cell may be correlated with an ohmic value of the battery cell, such as an internal resistance, internal impedance, and/or internal conductance of the battery cell. For example, an increased internal resistance, increased internal impedance, and/or decreased internal conductance of a battery cell may be used to detect an impending failure or an actual failure of the battery cell. A battery cell which has been detected to have the impending failure or to have failed may be replaced.
In this regard, FIG. 1 illustrates a battery monitoring system 10. The battery monitoring system 10 comprises a battery monitoring control unit 12 and a battery monitoring device 14. The battery monitoring device 14 is configured to test performance values of a battery 16. The performance values may comprise ohmic values 17 of battery cells 18 of a backup power supply provided in the form of the battery 16. The battery 16 is comprised of a plurality of battery cells 18(1)-18(4) electrically connected in series. Each battery monitoring device 14 may be coupled to a subset 19 of battery cells 18 electrically connected in series and in a sequential order to form the battery 16. The subset 19 may comprise a battery cell substring with a unique set of battery cells 18 in the battery 16. The battery monitoring device 14 provides a pair of current-inducing leads L1, L2 configured to be coupled to the negative and positive terminals of a battery cell substring of the battery 16. The battery monitoring control unit 12 may instruct the battery monitoring device 14 to produce a current through the subset 19 of battery cells 18 (as a non-limiting example, battery cells 18(1)-18(4)) by activating a switch to place a resistive load in a current loop with the subset 19 of battery cells 18 (as a non-limiting example, battery cells 18(1)-18(4)) of the battery 16.
The battery monitoring device 14 further provides a plurality of voltage sensing leads V1-V5. The voltage sensing leads V1-V5 are configured to be coupled to measure a voltage across the negative and positive terminals of each battery cell 18(1)-18(4). As illustrated in FIG. 1, voltage leads V1-V5 have resistances RV1-RV5 and the pair of current-inducing leads L1-L2 have resistances RL1-RL2. To increase the accuracy of measured voltages, the battery monitoring device 14 may employ Kelvin sensing. In this regard, voltage sensing leads V1, V5 may optionally be provided separate from the current-inducing leads L1, L2 allowing the measured voltages to be more accurate than a system in which a single lead is used for both L1 and V1 and another single lead is used for both L2 and V5. This is because separating the current-inducing lead L1 from the voltage sensing lead V1 and separating the current-inducing lead L2 from the voltage sensing lead V5 significantly reduces the impedance contribution of the voltage sensing leads V1, V5. Because there is almost no current flow in the voltage sensing leads V1, V5, the voltage drop across the voltage sensing leads V1, V5 (i.e., across RV1 and RV5) is lower. As a result, using separate current-inducing leads L1, L2 and voltage sensing leads V1, V5 enables a more accurate measurement of the voltages across the battery cells 18(1)-18(4).
The battery monitoring device 14 may test an ohmic value 17 of a battery cell 18 by inducing a current through the subset 19 of battery cells 18 assigned to the battery monitoring device 14. The battery monitoring device 14 may induce the current at a predetermined frequency for a predetermined period of time. As a non-limiting example, the current may draw a predetermined amount of current from the subset 19 of battery cells 18. This may allow the battery monitoring device 14 to discriminate effects of the measurement from the noise generated by other loads pulling current from the battery 16 and/or generator(s) charging the battery cells 18 of the battery 16.
As discussed above, it is important to identify the battery cells 18(1)-18(4) in the battery monitoring system 10 that are either failing or underperforming. In this regard, the SOH information about each of the battery cells 18(1)-18(4) can be monitored through automated ohmic testing of the battery cells 18(1)-18(4). The battery monitoring control unit 12 can be configured to compare the ohmic test results of the battery cells 18(1)-18(4) to a predefined ohmic threshold. The battery cells 18(1)-18(4) identified as not meeting the predefined ohmic threshold can be designated as failing or underperforming. In response, the battery monitoring control unit 12 can be configured to generate alarms identifying the failing or underperforming battery cells 18(1)-18(4) so that technicians can be warned. The failing or underperforming battery cells 18(1)-18(4) can be replaced to ensure sufficient back-up power, when needed.
The predefined ohmic thresholds must be established for the battery monitoring control unit 12 to generate alarms. The predefined ohmic thresholds can be established through ohmic threshold settings. The ohmic threshold settings can be manually entered by a technician into the battery monitoring control unit 12 for each battery cell 18(1)-18(4). However, manual entry relies on human behavior. If a technician does not establish ohmic threshold settings, the battery monitoring control unit 12 may use the same, default ohmic threshold setting for each battery cell 18({1)-18(4). But, because different battery cells 18 can vary in their ohmic performance characteristics, a default ohmic threshold setting may not be accurate for each battery cell 18(1)-18(4). Also, other factors can affect the ohmic performance characteristics of a battery cell 18 that are not taken into consideration by a technician when providing ohmic threshold settings. For example, the connections (i.e., links) of battery cells 18(1)-18(4) is can also affect ohmic performance characteristics that may not be properly taken into account by a technician.